1. Field of the Invention
This invention relates to cooling of turbine airfoils and more particularly to hollow turbine vanes having double shell airfoil walls.
2. Description of Related Art
It is well known to cool parts using heat transfer across walls having hot and cold surfaces by flowing a cooling fluid in contact with the cold surface to remove the heat transferred across from the hot surface. Among the various cooling techniques presently used are convection, impingement and film cooling as well as radiation. These cooling techniques have been used to cool gas turbine engine hot section components such as turbine vanes and blades. A great many high pressure turbine (HPT) vanes, and particularly the high pressure turbine inlet guide vane, also known as the combustor nozzle guide vane, utilize some form of a cooled hollow airfoil. An airfoil typically has a hollow body section which includes a leading edge having a leading edge wall followed by a pressure side wall and a suction side wall which form a substantial part of the outer wall which includes the hot wetted surface on the outside of the walls. The pressure and suction side walls typically converge to form a trailing edge.
Typically, a vane having a hollow airfoil is cooled using two main cavities, one with coolant air fed from an inboard radial location and the other with coolant air fed from an outboard location. These cavities contain impingement inserts which serve to receive cooling air and direct the coolant in impingement jet arrays against the outer wall of the airfoil's leading edge and pressure and suction side walls to transfer energy from the walls to the fluid, thereby, cooling the wall. These inserts are positioned by inward protrusions from the outer wall of the airfoil. These protrusions or positioning dimples are not connected to the inserts and provide the barest of contact between the insert and the airfoil wall (no intimate material contact at all). The high pressure of the cooling air in the cavity or insert is greater than that of the air on the outside of the airfoil causing a great deal of stress across the airfoil wall. One of the most frequent distress and life limiting mechanisms in conventional and particularly single wall vane airfoils is suction side panel blowout. This is a creep rupture phenomenon caused by stresses due to bending and temperature. Therefore an airfoil design is needed that will reduce these stresses and prolong the creep rupture life of the airfoil and turbine vane or blade.
Disclosed in U.S. Pat. No. 3,806,276 entitled "Cooled Turbine Blade", by Aspinwall, is a turbine blade having an insert or a liner made of a high conductivity metal such as cuprous nickel and which is bonded to a point on the radially extending ribs along the outer wall of the blade. The liner, because it is made of a high conductivity metal such as cuprous nickel has low strength and must be considered as dead load (non load/stress carrying). Therefore, it adds no significant stiffness to the airfoil and is not very capable of resisting bending moments due to the pressure differential across the airfoil outer wall. Another drawback is the bond points because they are inherently weaker than the surrounding material and therefore subject to failure under loads due to pressure differential induced bending moments and centrifugal forces in the case of rotating blades. Furthermore, since the insert is dead load, the outer wall of the blade will have to be thickened to carry the additional mass due to the centrifugal load which a turbine blade is subjected to. This will effectively increase the temperature differential .DELTA.T across the outer wall thereby raising the peak surface temp and the thermal stresses.
Such vanes also utilize other common design features for cooling such as film cooling and a trailing edge slot and have typically been manufactured from materials with thermal conductivities in the range of 10 to 15 BTU/hr/ft/.degree. F. A primary goal of turbine design is improved efficiency, and a key role in this is the reduction of component cooling flows. With the development of intermetallic materials, thermal conductivities on the order of 40 BTU/hr/ft/.degree. F. or even greater may be realized. Fabrication of intermetallic components by means other than casting or welding allows the design of more complex components with new features.
Turbine vane cooling requires a great deal of cooling fluid flow which typically requires the use of power and is therefore generally looked upon as a fuel efficiency and power penalty in the gas turbine industry. The present invention provides improved turbine vane cooling and engine efficiency.